Certain devices such as wafer defect scanners, laser printers, document scanners, projectors and the like make use of a narrow collimated laser beam that is usually scanned across a flat surface along a straight line path. A typical optical scanning system for this purpose employs a tilting flat mirror to deflect the beam. The tilting micro-mirror serves as a central element in many Micro Electro Mechanical Systems (“MEMS”) devices. It typically comprises a flat plate (e.g. made of silicon) having a reflecting surface. The plate is held suspended by two torsional hinges aligned along a common torsion (and tilt) axis or a gimbal. The two hinges allow the mirror to tilt clockwise and counterclockwise within a given range of angles, and a collimated laser beam that impinges on the reflecting surface of the mirror is redirected by the mirror to a projection area. The tilting mirror is actuated by an actuation moment that can be provided by well-known MEMS actuation systems. The collimated input beam is aimed perpendicular to the scanning mirror's rotational axis, so that the main deflected ray sweeps a plane in space. Beam collimation generally ensures that the spot size remains substantially the same at both the center and edges of the flat surface.
However, one of the problems associated with the operation of a MEMS type of mirror is how to confirm that the mirror is indeed oscillating, as a failure that causes the mirror(s) to stop oscillating would lead to safety hazards caused by the fact the all of the laser energy that should have been divided at the projection plane will now be focused onto a very small focal point. In addition, certain applications require long term positioning accuracy such as in optical communication applications, e.g. in various optical switches and variable optical attenuators implemented in communication systems, and in light processing devices used in projection technology.
Several attempts were made to solve these challenges. Some of these attempts are described in the following publications.
U.S. Pat. No. 6,538,802 describes a movable MEMS mirror system with a mirror position detection system, such as a capacitive sensor, that is calibrated using a physical stop with a range of movement of the mirror structure. Thus, a drift in the position detection system can be compensated without the need for a separate reference signal source as used in conventional systems.
U.S. Pat. No. 7,252,394 discloses the use of a capacitance sensor embedded in a scanning mirror which is used to detect a change in capacitance from the movement of the reflecting mirror unit. Another alternative described therein is the use of a piezoelectric sensor embedded in a scanning mirror and used to detect a change in current from the movement of reflecting mirror unit.
U.S. Pat. No. 6,275,326 describes a microelectromechanical sensor used to sense the position of a movable element in a microelectromechanical system. The microelectromechanical sensor being a strain gage, a gage of a capacitive, piezoelectric, piezoresistive or pressure type, adjoins the movable element and the resulting signal is fed back to control the component for moving the movable element. In an array of movable elements and sensors, the signal of each sensor is specific to one movable element.
US 20070064300 teaches a method of using in a projection system a microphone for detecting non-soundtrack disturbances, such as sounds or vibrations, generated exteriorly of the projection device, such as from household appliances, people walking in close proximity to projection system, etc. In operation, a signal generator generates a driving square wave signal. The signals are received at a filter from an audio amplifier and contain one or more frequency components that either act to produce the unwanted oscillatory displacements of optical component and/or are the result of non-audio-component sources.
However, there is still a need for a solution which can ensure that the MEMS structure is oscillating as required and will allow determining the phase of these oscillations to ensure that there is no deformation in the mirror's movement which in turn would lead to the distortion of the image that will be projected onto the projection plane.